Introduction: Acacia tortilis (Forssk.) Hayne is a key species of tree in the scorching hot and dry regions of Iran; its natural habitat is entirely limited to Hormozgan Province. Understanding the quantitative and spatial structure of A. tortilis is crucial for sustainable management, preservation and long-term monitoring of this species. The aim of this paper is to find out the structural properties of the natural stands of this species and create a baseline database for future monitoring of the species in Homeyran and Kong regions of Bandar Lengeh County, southern Iran.
Method: For this purpose, two one-hectare sample plots were established, and the following variables were measured for all trees: collar diameter, height, crown area, number of shoots, stand density, natural regeneration, and spatial position. In addition, the spatial and quantitative structure of the stands was assessed using Crancod 3.1 software and nearest-neighbor-based structural indices, including neighborhood distance, uniform angle index, diameter differentiation, height differentiation, diameter dominance, mingling, and structural diversity.
Results: According to the results, the number of living trees was (49 and 60 ha⁻¹ in Hamiran and Kong respectively). On the other hand, the density of regeneration was almost three times greater in the case of Homeyran (60 saplings ha⁻¹) than in Kong (22 saplings ha⁻¹), thus indicating a better potential for natural regeneration there. Nevertheless, Kong demonstrated higher values in terms of mean basal area (16.49 cm), tree height (3.58 m), and crown area (42.88 m²) as well as substantially better canopy cover (72.25%) in comparison to Homeyran (39.50%). Among the percent of dead trees Homeyran had many more of them (30.06% versus 8.33% for Kong) suggesting that the site may be less favorable or may have experienced greater environmental stress compared to the latter. The distributions for diameter and height are irregular for both habitats of trees with the biggest number of trees being in the lower and intermediate classes which means that there are more young trees than the old ones. The mean distance between the nearest neighboring tree is equal to 4.83 m and 4.99 m for Homeyran and Kong correspondingly while the nearest neighbor’s index is equal to 8.95 m and 8.79 m for these two stands respectively. Results from the Clark and Evans index showed that Homeyran exhibited a clustered spatial distribution while Kong showed a nearly random pattern leaning towards regularity, and the uniform angle index (0.62 and 0.51) indicated that both stands have an aggregated distribution of trees. Diameter differentiation (0.67 and 0.51) and height differentiation (0.50 and 0.34) are also relatively substantial and pronounced in Homeyran than in Kong meaning that Homeyran is more structurally heterogeneous compared to Kong. The low structural diversity index (0.30 in Homeyran and 0.25 in Kong) and a zero mingling index also confirmed that both stands have a rather simplified, mono-specific and homogeneous structure.
Conclusion: The results revealed significant differences in the quantitative and spatial structure of natural Acacia tortilis stands between Homeyran and Kong. Despite exhibiting higher natural regeneration, the Homeyran stand showed greater tree mortality and a more aggregated spatial pattern, whereas the Kong stand was characterized by larger trees, higher canopy cover, and relatively greater stand stability. These findings provide valuable baseline information for the long-term monitoring, conservation, and sustainable management of natural A. tortilis stands in southern Iran.